This research will attempt to determine how many kinds of heat-stable enterotoxins (ST) exist with respect to chemical composition and biological activity. The amino acid sequence of porcine, human, and bovine STs will be determined in order to ascertain whether age and species susceptibility is related to differences in toxins or bacterial receptors. STB with biological activity in jejunal loops of piglets, but inactive in suckling mice, will be purified and chemically characterized. Further, it will be shown whether one strain of ENT positive E. coli can produce more than one kind of ST. The receptor for ST on the cell surfaces will be characterized and isolated using heterobifunctional crosslinking reagents. The mechanism of action of ST will be further characterized using intestinal membrane preparations, intestinal cells in culture and the rat perfusion model. Hypertoxigenic (HYP positive) mutants of ENT positive E. coli which exhibit increased production of heat-labile enterotoxin (LT) will be isolated and used to study the antigenic diversity of LTs from porcine and human strains. Each LT will be characterized by amino acid analysis, peptide mapping, and immunological properties using a solid-phase radioimmunoassay. The binding of iodinated LT to liposomes will be studied, and the membrane-receptor complex for LT characterized using crosslinking reagents. Finally, the mechanism of secretion and processing of LT by ENT positive strains of E. coli will be characterized.